(1) Field of the Invention
The present invention relates to a flux-cored wire for welding different materials for use in welding different materials, for example, transport machinery such as automobiles and components for building materials and the like, and a method for laser welding of different materials and method for MIG welding different materials using this flux-cored wire for welding different materials, especially to a flux-cored wire for welding different materials suitable for use in welding between aluminum or an aluminum alloy material and a galvanized steel plate, a method for laser welding of different materials and a method for MIG welding of different materials.
(2) Description of the Related Art
Heretofore, in transport machinery such as automobiles, steel materials are used as materials for vehicle bodies and the like. Since steel materials used as components of transport machinery and the like are exposed to rain water and the like during use, antirust galvanizing is performed on their surfaces. Accordingly, a waterproof effect is provided by an oxide film formed on the galvanized surface, while zinc is corroded (sacrificial protection) preferentially to iron even when there are flaws, pin holes or the like on the surface of a steel material.
Recently, from the perspective of protecting the environment, research and development of hybrid automobiles, electric automobiles and the like has been rapidly conducted, weight reduction of vehicle bodies and the like of these automobiles are required to improve fuel efficiency and for other purposes. Moreover, in order to achieve weight reduction of vehicle bodies and other parts, constituting part of steel materials used as materials by aluminum or aluminum alloy materials (hereinafter aluminum materials and aluminum alloy materials are collectively referred to as aluminum alloy materials) has been studied.
Therefore, producing vehicle bodies and the like requires different-material bonding between a steel material and an aluminum alloy material. Examples of different-material bonding techniques between a steel material and an aluminum alloy material include a method of bonding by MIG welding or laser welding between base materials while feeding a flux-cored wire (Japanese Unexamined Patent Publication No. 2008-68290).
In this prior art, the compositions of the outer sheath of the flux-cored wire is defined to be Si: 1 to 13%, and a fluoride-based flux containing no chloride is charged into the outer sheath at a packing fraction of 0.3 to 20% by mass.
In addition, in order to improve the bonding strength of a bonded structure of different materials, a filler metal in which the amount of Si contained is 1.5 to 6.0%, and further Zr is contained as an additive component in an amount of 0.1 to 0.2% by mass has been suggested (Japanese Unexamined Patent Publication No. 2006-224145).
MIG (Metal Inert Gas: MIG) welding is a method of feeding an inert gas such as argon or helium to the vicinity of a site to be bonded as a shielding gas and generating an arc between a welding wire and the site to bond a steel material and an aluminum alloy material. This MIG welding is characterized in that welding operation is carried out in a state of being blocked from the atmosphere, and therefore welding proceeds without being affected by oxygen in the air. In contrast, laser welding comprises feeding a welding wire to a joint portion while this welding wire and joint portion are irradiated with laser light so that the welding wire and the joint portion are thermally fused by the laser light.
However, the aforementioned prior art has the following problems: in a structure such as a vehicle body of an automobile, in the case where a steel material and an aluminum alloy material are butt-welded, tensile stress acts between base materials when an external force is applied to the welded joint portion. Meanwhile, in the case where, for example, a steel material and an aluminum alloy material are overlapped and welded, and when an external force is applied to the welded joint portion, tensile stress acts between the base materials, and at the same time peeling stress which pulls the two base materials away from each other acts on the weld interface. Therefore, when different materials are welded, not only tensile shear strength but also high peeling strength (peel strength) is required for the welded joint portion. However, when a steel material and an aluminum alloy material are bonded by welding as in a conventional case, a highly brittle intermetallic compound is produced at the bonded portion, which causes the problem that the tensile shear strength and peeling strength of the bonded portion are lower than in the case where the same type of components are welded.
In contrast, in the case where different-material welding is performed by using the flux-cored wire disclosed in Japanese Unexamined Patent Publication No. 2008-68290, production of this highly brittle intermetallic compound can be suppressed, and the thickness of the intermetallic compound layer can be reduced.
However, the tensile shear strength after welding is increased in an area where the amount of Si contained in the filler metal is high, but peeling strength is disadvantageously lowered. Moreover, although peeling strength is improved in an area with a low amount of Si and peeling does not occur in the intermetallic compound layer, when there is a large difference in heat expansion between the aluminum material and the steel material (for example, the case where the thickness of the aluminum material is larger than that of the steel material, etc.), cracks disadvantageously occur in the welded portion (welded metal portion) due to the thermal shrinkage of the welded portion.
In contrast, as in Japanese Unexamined Patent Publication No. 2006-224145, it is also possible to keep the amount of Si in a solid filler metal to a relatively low level of 1.5 to 6% and further add Zr as an optional component as appropriate.
However, when the amount of Si is relatively high, or depending on the main compositions and packing fraction of the flux in the flux-cored wire, formation of an intermetallic compound layer cannot be effectively suppressed, and high peeling strength cannot be obtained.